This invention relates to a switching power source device, and more particularly to a switching power source device in which a DC voltage obtained by rectifying a voltage from an AC power source is subjected to high-frequency conversion by switching elements consisting of transistors, the resulting voltage being amplified or attenuated to the required level by a transformer to obtain an AC output, which is further rectified and smoothed as necessary to obtain DC outputs.
There are various systems of switching power source devices adapted to switch a DC voltage obtained by rectifying a voltage from an AC power source by using switching elements. FIG. 1 shows an example of a half-bridge type of switching power source device using power MOSFETs (metal-oxide-semiconductor field effect transistors) as switching elements, and FIG. 2 shows the operating waveforms of parts of the device shown in FIG. 1. The switching power source device shown in FIG. 1 consists of a rectifier circuit 13 which changes an AC voltage supplied from an AC power source 12 into a DC voltage; a smoothing capacitor 1 which smooths the DC voltage output from the rectifier circuit 13; FETs 2, 3 which alternately switch the smoothed DC voltage at high speed; capacitors 4, to which the electric currents switched by the FETs are applied; a transformer 5, to which an electric current Iin flowing through the capacitors 4 is applied, the transformer 5 having a primary coil 5a, secondary coils 5c, 5d, and a core 5b magnetically connecting these coils; a plurality of diodes 8 rectifying the AC voltages output from the secondary coils 5c and 5d of the transformer 5; two output filter circuits to smooth the DC voltages output from the diodes 8 and output them as DC voltages Vout 1, Vout 2, each of the output filter circuits being composed of an inductor 9 and a capacitor 10; and a pulse width modulator circuit 11 which receives the DC voltage Vout 1 as an input thereinto to carry out the switching operations of the FETs 2, 3.
The switching power source device having the above construction operates as follows.
The AC voltage output from the AC power source 12 is converted into a smoothed DC voltage by the rectifier circuit 13 and smoothing capacitor 1. This DC voltage is converted into an alternating current Iin through the capacitors 4 by the FETs 2, 3, which are turned on and off alternately, as shown in FIG. 2, by the pulse width modulator control circuit 11. The AC current Iin is applied as an AC voltage Vin across the primary coil 5a of the transformer 5. The voltage Vin applied across the primary coil 5a is transformed by the core 5b of the transformer 5 and output to the secondary coils 5c, 5d. The AC voltages output from the secondary coils 5c, 5d are smoothed by the diodes 8 and output filter circuits and are output as DC voltages Vout 1, Vout 2, respectively. The pulse width modulator control circuit 11 receives the DC voltage Vout 1 as an input to control the width of the pulses switching on and off the FETs 2, 3 in such manner that the DC voltage Vout 1 attains a predetermined level. A switching power source device similar to the above device is disclosed in U.S. Pat. No. 3,916,286.
In order to reduce the switching losses of the FETs 2, 3 due to the modulation of the pulse width, it is necessary in the above switching power source device that each voltage and current rises or falls suddenly. However, when a voltage or current suddenly rises or falls in this device, noise occurs during the switching of the FETs 2, 3. When a voltage suddenly rises or falls in this device, losses due to the inverse recovery characteristics and noise occurs in the diodes 8 on the secondary side of the transformer 5. Furthermore, this device employs a miniaturized transformer to increase the frequency of the output therefrom, and a plurality of outputs are obtained from this transformer. Accordingly, it is difficult to connect the primary and secondary coils magnetically in a satisfactory manner.